(1) Field of the Invention
This invention relates to a process for preparing cyclic terpenoids of general formula (I), ##STR3## wherein R is a hydrogen atom or a lower alkyl group.
The cyclic terpenoids of general formula (I) are valuable as intermediates for the synthesis of vitamin A acid and vitamin A acetate which are in use of medicines and feed additives. (See Otera et al., J. Am. Chem. Soc., 106, 3670 (1984); Otera et al., J. Org. Chem., 51, 3834 (1986))
(2) Description of the Related Art
It has been reported that cyclic terpenoids are conventionally prepared by the following routes. ##STR4## (Yield of [2]+[3] was 93%. The formation ratio of [2]/[3] was 5/1. See Example 8 of Japanese Patent Publication No. 57-48549.) ##STR5## (Yield of [2]+[3]+[4] was 95%. The formation ratio of [2]/[3]/[4] was 86/9/5. See Uneyama et al., Bull. Chem. Soc. Jpn., 58, 1859 (1985).)
The conventional methods of preparing cyclic terpenoids are cyclization reaction of acrylic terpenoids under acidic conditions. As described above, the cyclic terpenoids necessary for the synthesis of vitamin A acid and vitamin A acetate are the compounds of general formula (I) (hereinafter referred to as .beta.-form). The terpenoids prepared by a conventional method contain .beta.-form and also cyclic terpenoids of general formula (II) (hereinafter referred to as .alpha.-form) as a by-product. ##STR6## wherein R is the same as defined above. Consequently the yield of .beta.-form at the cyclization becomes low, and a complicated process for the separation of by-product .alpha.-form such as silica gel chromatography and the like are needed.
Accordingly, an object of the invention is to provide a process for preparing .beta.-form, the starting material of vitamin A acid and vitamin A acetate, at a good yield in a commercial scale.
Other objects, features and advantages of the invention will become apparent from the following description.